Method of making hydrogel laminates

ABSTRACT

A COMPARATIVELY THICH SHEET OF A SPONGY HYDROGEL, MADE E.G. BY POLYMERIZING ETHYLENE GLYCOL MONOMETHACRYLATE IN PRESENCE OF LESS THAN 2 PERCENT OF A CROSS-LINKING AGENT AND MORE THAN 60 PERCENT OF WATER, IS LAMINATED WITH A MUCH THINNER SHEET OF THE SAME OR SIMILAR, BUT HOMOGENEOUS HYDROGEL, EITHER FROM ONE SIDE OR FROM BOTH SIDES. ALTERNATIVELY, ONE EXTERNAL LAYER MAY BE LESS HYDROPHILIC OR FULLY HYDROPHILIC. THE LAMINATION IS CARRIED OUT SO THAT THE EXTERNAL THIN LAYER IS MADE ON A GLASS OR OTHER SMOOTH PAD, EITHER BY POLYMERIZING A CROSS-LINKED OR LINEAR POLYMER THEREON, OR BY CASTING A THIN LAYER OF A DISSOLVED LINEAR POLYMEER. THEN A MONOMER MIXTURE FOR SPONGY HYDROGEL IS CASTT ONTO THE FIRST LAYER AND POLYMERIZED. A SANDWICH STRUCTURE IS OBTAINED IF THE MONOMER MIXTURE FOR SPONGY HYDROGEL IS CAST BETWEEN TWO PADS PROVIDED WITH A THIN HOMOGENEOUS POLYMER LAYER. HYDROGEL LAMINATES AND SANDWICH STRUCTURES ARE USEFUL E.G. FOR SURGICAL AND COSMETIC PURPOSES, FOR FILTERING LIQUIDS ETC.

U.S. Cl. 156-242 "United States Patent 3,825,458 METHOD OF MAKINGHYDROGEL LAMINATES Otto Wichterle and Karel Kliment, Prague,Czechoslovakia, assignors to Ceskoslovenska Akademie ved, Prague,Czechoslovakia No Drawing. Original application Feb. 25, 1969, Ser. No.808,202, now abandoned. Divided and this application Nov. 22, 1971, Ser.No. 201,235

Claims priority, application Czechoslovakia, Mar. 6, 1968, 1,767/ 68Int. Cl. B29d 7/02 13 Claims ABSTRACT OF DISCLOSURE A comparativelythick sheet of a spongy hydrogel, made e.g. by polymerizing ethyleneglycol monomethacrylate in presence of less than 2 percent of across-linking agent and more than 60 percent of water, is laminated witha much thinner sheet of the same or similar, but homogeneous hydrogel,either from one side or from both sides. Alternatively, one externallayer may be less hydrophilic or fully hydrophobic. The lamination iscarried out so that the external thin layer is made on a glass or othersmooth pad, either by polymerizing a cross-linked or linear polymerthereon, or by casting a thin layer of a dissolved linear polymer. Thena monomer mixture for spongy hydrogel is cast onto the first layer andpolymerized. A sandwich structure is obtained if the monomer mixture forspongy hydrogel is cast between two pads provided with a thinhomogeneous polymer layer. Hydrogel laminates and sandwich structuresare useful e.g. for surgical and cosmetic purposes, for filteringliquids etc.

This is a division of application Ser. No. 802,202, filed Feb. 25, 1969,now abandoned.

The U.S. Patent Specifications Nos. 2,976,576 and 3,220,960 disclosesparingly cross-linked hydrophilic polymers, particularly such derivedfrom glycol methacrylate and glycol acrylate polymers, polymerized inpresence of less than 2% of a cross-linking agent and in presence ofwater or water-miscible inert liquids such as glycols or glycerol. Inpresence of less hydrophilic co-rnonomers, or in presence of excessiveamounts of water, in which the copolymer thus formed swells but to alimited degree so that the liquid phase separates in fine dropletsduring the polymerization, a spongy, porous structure is formed whichmay be utilized for various purposes, particularly for makingprostheses, or for filtering etc.

In surgery, the prostheses made from said spongy hydrogels are usedbeing first saturated with physiologic solution, and, if desired, withantibiotics or other physiologically active substances. If the pores arelarger than about 100 and particularly if they are filled up withcollagen (according to the U.S. Patent application No. 691,947 now U.S.Pat. 3,563,925, dated Feb. 16, 1971) the neighboring tissue grows intothe pores and the spongy hydrogel prosthese is firmly bound to thetissue. In some cases, however, a part of the surface should remain freeso that the growth of the tissue into such part is avoided.

Hydrogels of the above mentioned kind may be also used as carriers forphysiologically active substances for internal or external application,according to the U.S. Patent application No. 607,637 now U.S. Pat.3,551,556, dater Dec. 29, 1970. In said carriers, the controllablediffusion of drugs through a defined layer of the hydrogel makes itpossible to secure a locally increased concentration for a predeterminedtime. Hydrogel sponge is capable of retaining very large amounts ofdrugs, the biologically active substance is, however, liberatedtherefrom too rapidly when in direct contact with living body. An exact3,825,458 Patented July 23, 1974 "ice control of the treatment is thenimpossible. It was thus proposed to use spongy hydrogel as a coreimbided with a physiologically active substance, onto which a layer ofhomogeneous cross-linked hydrogel is then cast by polymerization. Thismethod has, however, certain shortcomings. The liquid monomer mixturepenetrates irregularly into the pores so that the homogeneous externallayer possesses a very uneven thickness. Moreover, some drugs areunstable in contact with polymerization initiators and activators oreven monomers. These circumstances impair the accuracy of doses and makecontrol difficult.

Similar shortcomings could be observed when using spongy hydrogels assurgical dressing material. The penetrability is sometimes too high sothat the possibility of an infection cannot be excluded, the dressingdries too rapidly or the drug or disinfectant, incorporated therein, isliberated in an uncontrollable way.

All of the above shortcomings of spongy hydrogels can be avoided bylaminating them according to present invention with a thin layer or ahomogeneous polymer, particularly hydrogel. The lamination is carriedout by making the thin external layer separately on a smooth pad, e.g. aglass plate, either by polymerization-casting or by solution-casting.The subsequently polymerization-cast spongy hydrogel is perfectly boundwith the external layer since some monomer penetrates into the latterand secures the bond by polymerization and chain-transfer reactions. Thethickness of the homogeneous external sheet remains, however, unchanged,the biologically active substance penetrates its uniform layer evenly,the growth of the surrounding tissue into the implantate is avoided, theevaporation of water controlled and the penetration of microbes andviruses made impossible. The external thin sheet can be composed, ifdesired, of two layers having different properties, particularly if theouter layer should be fully hydrophobic. Then, an intermediate layerwhich is less hydrophilic than the hydrogel but warranting a firm bondbetween the latter and the hydrophobic outer layer may be useful.

In order to obtain even, non-corrugated outer layers, it is necessary tomaintain the swelling degree of the two layers, of the homogeneous andthe spongy one, on the same level, advantageously such as will be duringthe use of the laminate in equilibrium with the surrounding medium.

For building thin homogeneous layers, soluble, noncross-linked glycolmethacrylate or acrylate polymers, prepared according to the U.S. Patentapplication No. 639,021 now U.S. Pat. 3,575,946, dated Apr. 20, 1971,may be used. Choosing a proper solvent and polymer concentration makespossible to gain very thin and uniform foils. After evaporating thesolvent a monomer mixture for spongy hydrogel is cast onto the foil.Since the hydrogel foil is swelling in the monomer mixture, the monomerspenetrate into the thin foil which is then covalently bound to thespongy hydrogel by entangled macromolecules and by grafting the growingchains onto the polymer of the foil. The cross-linking agent, containedin the monomer mixture provokes cross-linking of the soluble glycolmethacrylate or acrylate polymer which contains some vinyl sid groups.As a result, the originally soluble polymer of the outer foil becomesinsoluble.

The use of soluble glycol methacrylate or acrylate polymers instead ofpolymer-cast layers has the advantage of enabling much simplerprocessing and broader versatility, the solutions for casting the foilbeing preparable in a wide range of viscosities and average molecularweights in mixtures of solvents having different volatility.

By washing in water or in another swelling liquid the whole laminate iseasily separated from the plate on which the outer homogeneous foil hasbeen prepared.

Washed laminated or sandwich structure is then sterilized by boiling andkept in a sterile physiologic solution containing, if desired,antibiotics, disinfectants or other preservation means or drugs.

A sandwich structure is made in the same way, only the monomer mixturefor spongy hydrogel is cast between two plates provided on their innersides by a thin homogeneous foil of the same or other polymer.

Large laminated sheets may be cut to pieces of appropriate shape andsize. The side surfaces of the hydrogel sponge may be made tight, ifdesired, either by dipping them into a polymerizing mixture forhomogeneous hydrogel, or by bonding them with another thin foil in theway described above, the initiated monomer mixture for spongy hydrogelbeing preferably used as adhesive. If desired, however, any othersutficiently hydrophilic adhesive may be used for said purpose.

The smooth pad, on which the thin homogeneous foil is prepared, may beremoved, after finishing the laminate or sandwich structure, either bysimple stripping, preferably in swollen state, or also by dissolving itin a suitable agent. For instance, a glass plate can be removed bydissolving it in diluted hydrofluoric acid and washing thefluorosilicates out from the laminate. This method is advisable in caseswhere the laminate has a rather complex shape, the glass pad being soenclosed in the hydrogel body that it could not be removed otherwisewithout damaging the laminate. An example is a pad consisting of asheet, a fibre or a rod of glass, particularly a bent one. Thehydrofluoric acid diffuses through the outer spongy layer and the innerhomogeneous hydrogel layer and dissolves the glass. Preferably specialglasses are used for this purpose, leaving no insoluble residue afterhaving been decomposed by hydrofluoric acid. This special embodiment ofthe invention is a novel modification of the process disclosed inExample 2 of the US. Patent Specification No. 2,976,576 of the sameinventor, wherein, however, no laminates with spongy core andhomogeneous outer layer were mentioned.

The outer layer may be reinforced, if desired, by a thin fabric orknitted fabric made from suitable, particularly synthetic fibres such aspolyethylene terephthalate. Similar reinforcing inserts may be used forstrengthening the spongy hydrogel layer.

The laminates according to the invention may be manufactured eitherdiscontinuously or continuously. For batchwise manufacture it ispossible to use, for example molds in the form of glass orchromium-plated metal sheets provided with a side-wall of appropriateheight. For laminates provided on both sides with a homogeneous layertwo such molds are combined, with elastic sealing means between them.

Continuous manufacture may be carried out on a conveyor formed fromappropriate non-adhesive plastic such as polytetrafluorethylene on whichis first provided with a thin layer from homogeneous polymer, e.g.soluble ethylene glycol methacrylate polymer dissolved in aqueous ethylalcohol. An already polynnerizing monomer mixture for the spongyhydrogel is cast onto the dried thin layer. The conveyor is moved into aspace filled with inert gas and kept at suitable temperature at whichthe polymerization to high conversion is finished during severalminutes. The time and temperature of the polymerization depends on theinitiating system used. Redox systems such as soluble persulfates withreducing substances and traces of iron or copper ions are apt to inducea more rapid polymerization than persulfates alone or azo-initiators(e.g. azo-bis-isobutyronitrile). The conveyor is preferably tilted inthe direction of proceeding polymerization so that the alreadypolymerized mixture bars the way to the still liquid viscous mixturewhich cannot overflow. The sides of the conveyor are provided withelastic raised borders or otherwise secured against overflowing of themixture.

The thin homogeneous film can be also led through the polymerizationspace in the form of a separate foil, laid freely onto the conveyor.

Instead of an endless conveyor, a series of long troughs may be usedwith the homogeneous thin layer formed on their bottom can beintermittently or continuously transported through a polymerizationspace, being laid transversely onto one or more conveyors.

In continuous manufacture, the viscosity of the monomer mixture isincreased either by starting the polymerization prior to the casting butcasting prior to attaining the gel point, or by adding soluble polymer,e.g. such made according to the US. Patent Application No. 639,021, orby both of said measures. The first way involves the use of so calledpre-polymers, the polymerization of which was discontinued for certaintime, e.g. by cooling down. Another means for increasing the viscosityis admixture of finely powdered fillers such as silicium dioxide. Aparticularly suitable filler consists of finely ground waste sparinglycross-linked hydrogel of the same or similar kind as formed bypolymerization of the monomer mixture for spongy hydrogel. Such fillerswells considerably in the monomer mixture and strongly increases theviscosity thereof. It is then bound with the spongy hydrogel by numerouscovalent bonds caused by chain transfer as well as by entangling thegrowing chains into the polymer structure of the filler.

The laminate can be removed from the pad e.g. by immersing the wholeinto a liquid in which the hydrogel is swelled more than in waterpresent in the monomer mixture. Such liquid may be for instance ethylalcohol. In this way the laminate, adhering by its outer homogeneouslayer firmly to the pad, is loosened from the latter and simultaneouslythe soluble residues of the initiating system are washed out. Theseparated laminate is then washed in water, sterilized and immersed intoa physiologic solution (i.e. an aqueous electrolyte solution which isisotonic with liquids of the human body, e.g. a 0.8% sodium chloridesolution).

Laminates of the invention are valuable where a soluble substance has topenetrate by diffusion from the spongy hydrogel outward through a thinhomogeneous layer. If the diffusion has to be slow, an appropriatepolymer is used for building the outer layer, allowing the substance inquestion to diffuse at a suitable rate. For instance in forming anarticle for replacing the skin, the outer layer must be chosen so as toavoid rapid evaporation of water. A thin layer of butylrnethacrylate orsimilar polymer may be useful in such a case, covered by an intermediatelayer of a glycol methacrylate-methyl methacrylate or similar copolymer.

The term glycol used throughout the specification means not onlyethylene glycol, but also diethylene glycol, triethylene glycol andtheir mixtures, as well as propylene and butylene glycols and theirco-polycondensates with ethylene glycol.

The respective acrylate polymers are a bit more hydr0- philic and lessstable, but their stability is quite suflicient for surgical and mostother purposes.

From various hydrogels the glycol methacrylate and acrylate polymersproved best, until now, because they are very well tolerated by livingbody. Nevertheless, they may be partly or wholly replaced by similarpolymers and copolymers based on other hydrophilous monomers such asacrylamide, methacrylamide, copolymers of acrylonitrile with sodiummethacrylate or acrylate etc. As cross-linking agents glycolbis-methacrylates and acrylates are most suitable, being usually presentin purified monoesters in appropriate concentration, i.e. less than 2percent. Other suitable cross-linking agents are e.g. N,N'-methylene-bis-methacrylamide, triacryloyl perhydrotriazine and similar.Any sparingly cross-linked hydrophilic polymer which is physiologicallyinert and sufliciently stable can be used for the purpose of theinvention.

All parts and percentages in the specification and Examples are meant byweight, if not stated otherwise.

EXAMPLE 1 A solution of soluble, non-cross-linked ethylene glycolmethacrylate polymerin a mixture of 80% of ethanol and 20% of Water wascast onto a horizontal glass plate provided with a tight frame. Theamount of the cast solution was chosen with respect to the size of theplate so as to obtain, after evaporating the solvent, a 0.1 mm. thinpolymer layer. The solvent was evaporated at room temperature in orderto obtain smooth, transparent glossy surface. Then the plate was heatedto 60 C. and following monomer mixture was poured onto the polymerlayer: 30% of ethylene glycol monomethacrylate (containing 1.0% ofbis-methacrylate) and 70% of a 10% solution of ammonium persulfate indistilled water. The mold was then covered by another glass plate underwhich carbon dioxide was led in. Polymerization was finished in minutes.The mold was cooled down to room temperature, the spongy polymer spreadwith water and after an hour slowly stripped from the plate. The bondbetween the spongy layer and the homogeneous foil was very strong andthe polymer of the homogeneous foil became absolutely insoluble in allsolvents, in which the polymer readily dissolved before.

EXAMPLE 2 Thin foils (0.03 to 0.05 mm.) Were formed on two glass platesby polymerizing a mixture of ethylene glycol I monomethacrylate,containing 0.3% of ethylene glycol bis-methacrylate, by adding 0.2% ofdi-isopropyl percarbonate and covering the plates with oxygen-free gas(carbon dioxide). The mixture was degasified prior to having poured itonto the glass plates, heated to 65 C. After a while the mixturegelified. It was then covered by a polyethylene foil and by'the otherplate, so that the polyethylene foil was between the two polymerizinglayers. After further minutes the polymerization was finished, theplates were cooled down to the room temperature, the two platesseparated and the polyethylene foil carefully removed.

Then a mold was formed by clamping the two plates together, with thehydrogel layers inside and a distance strip from silicone rubber (2 mm.thick) between them. The mold was then immersed into a water-thermostatheated to 60 C., filled up with a monomer mixture for spongy hydrogel ofthe composition stated in Example 1 and left to polymerize for 20minutes. Then the distance strip was removed and the mold immersed intodistilled water. After several hours the swollen sandwich might beeasily separated from the glass plates.

EXAMPLE 3 Glass fibre bundle, diameter of single fibre 40-50 totaldiameter of the bundle 15 to 20 mm., length 100 to 120 mm. was firstpolymerized into hydrophilic polymer at its one end, by inserting 5-8mm. of the bundle into a glass tube, inner diameter -28 mm., usingmonomeric mixture of the following composition: 80% of ethylene glycolmonomethacrylate, containing 0.2% of the diester, and 20% of methylmethacrylate. As initiator, 0.3% of tert. butyl peroctoate was added.The polymerization was'carried out at 60 C. under inert gas. Care wastaken to maintain the fibers in parallel position. When one end of thebundle was anchored in a polymer block, the other end was treated in thesame way. The fiber bundle with both ends thus fixed was then repeatedlydipped into an 8% solution of soluble ethylene glycol methacrylatepolymer in methanol so as to secure coating all fibers with a thin layerof the polymer. After evaporating the solvent the bundle with the twofixed ends was put into a test tube having diameter about 1 mm. largerthan htat of the fixed ends. A monomer mixture for spongy polymer,consisting of 20% ethylene glycol methacrylate, containing 0.8% of thediester, and of a 10% ammonium persulfate aqueous solution, was pouredinto the test tube which was then degasified by applying several timesreduced pressure of a water jet air-pump. The temperature ofpolymerization was 60 C. After 30 minutes the spongy rod was carefullyremoved from the test tube, the two head surfaces cut off in order touncover the fiber ends and the whole was immersed for 5 hours into a 35%hydrofluoric acid. The spongy set of parallel capillaries was thenthoroughly washed in distilled water and inserted into a perforatedtube. It could be used for filtration.

EXAMPLE 4 A pre-polymer Was prepared by careful heating a mixtureconsisting of 60% of ethylene glycol monomethacrylate, containing 0.2%of the diester, and 40% of a mixture of 1,2 and 1,3-glycerol diacetate,adding 0.05% of dibenzoyl peroxide as initiator of polymerization. Thepolymerization was discontinued as soon as the refractive index of theprepolymer attained the value n =l.4670'. The prepolymer bad aconsistency suitable for casting or coating for continuous manufactureof laminates. After having admixed further 0.05% of di-isopropylpercarbonate and degasified the prepolymer polymerized to highconversions at 65 C. during 10 to 15 minutes. By adding more initiatorthe polymerization time can be reduced up to about 2 minutes.

The pregel was used for casting a thin foil on a glass plate. Themonomer mixture for spongy hydrogel according to Example 1 waspolymerized at room temperature onto the foil during about 8 hours.

EXAMPLE 5 A grit made from waste, dried, sparingly cross-linked hydrogel(made originally by polymerizing 60% of ethylene glycol monomethacrylatecontaining 0.2% of the diester, 20% of diethylenglycol monomethacrylate,containing 0.3% of the respective diester, 0.25% of di-isopropylpercarbonate and 19.75% of water, grain size 0.10 to 016p, was swelledfor 3 hours in a mixture of ethylene glycol monomethacrylate, containing0.3% of the diester. Then the mixture was thickened by adding 2% ofpowdered soluble non-cross-linked, ethylene glycol methacrylate polymerand polymerized under inert gas with 0.2% of di-isopropyl percarbonateat 60 C. in a thin layer on a chromium-plated metal sheet. Onto thepolymerized foil the initiated monomer mixture for spongy hydrogelaccording to Example 3 was poured and polymerized. The bond between thetwo layers was very strong.

EXAMPLE 6 A 5% solution of polyvinyl acetate (average molecular weight55,000) in acetone was cast onto a horizontal glass plate and left toevaporate at the room temperature to a 0.03 mm. thick foil. Onto thisfoil, another 1 mm. thick even layer of the prepolymer according toExample 4 was spread, initiated by 0.05% of diisopropyl percarbonate.The layer of the prepolymer was covered with a polyethylene foil andleft to polymerize at 60 C. The 1 mm. thick layer of the hydrogel isoverlaid by a 5 mm. thick layer, consisting of 30 parts (by weight) ofethylene glycol monomethacrylate, 0.1 p. of ethylene glycoldimethacrylate, 60 p. of a 1% solution of ammonium persulfate indistilled water and 8 p. of dry powdered soluble ethylene glycolmonomethacrylate polymer. When a homogeneous mixture has been formed,1.9 p. of dimethylaminoethyl acetate is added while stirring and themixture is brought onto the foil mentioned above and left to polymerizeunder an inert gas to a spongy polymer.

In similar way there is possible to create various outer layers, evenfrom quite hydrophobic polymers such as polyisobutylene, if a rapiddrying out of the laminate is to be avoided. A less hydrophobicintermediate layer could be interposed, as mentioned above.

EXAMPLE. 7'

. A solution of parts of sodium polymethacrylate and 5 p. of glycerol in80 p. of water was poured onto a horizontal paper pad 1 mm. thick, anddried freely on the air. A one percent ethanolic solution of polyvinylacetate, average molecular weight 10,000, was poured in a 0.5 mm. thicklayer onto the pad and freely evaporated at room temperature. Furtherlayer was formed by evaporating a 0.5 mm. thick layer of a ethanolicsolution of soluble, non-cross-linked polymeric ethylene glycolmethacrylate. Then a 2 mm. thick layer of spongy hydrogel waspolymerized onto the last layer, using the monomeric solution accordingto Example 1. After swelling the whole in water, the paper pad could beeasily re moved, leaving a laminate with very thin hydrophobic outerlayer and spongy core.

EXAMPLE 8 The process according to Example 7 was repeated with thedifference that a chromium plated smooth metal sheet was used as thepad, onto which a thin polyethylene terephthalate knitted fabric wascemented with a self-curing silicone rubber prepolymerizate. After thecuring finished a very thin film of poly(butylmethacrylate) varnish wasapplied to, and finally a 2 mm. thick layer of spongy hydrogel asdescribed in Example 1 was formed.

EXAMPLE 9 The process according to Example 3 was repeated, except thatthe glass fibers were coated previously, during the spinning process,with a thin layer of soluble glycol methacrylate polymer. Thepreparation of the fibers made easier the textile treatment such aswinding up to skein etc.

The soluble glycol methacrylate polymer may be prepared either by simplepolymerization of a pure monoester (see e.g. the Czechoslovakian Pat.No. 94,363), or by polymerization of a diester-containing monoester inpresence of a good solvent according to the US. Patent application No.639,021, avoiding the cross-linking. Such polymer is soluble e.g. in a50% aqueous ethanol, but fully insoluble in water or in liquids of thehuman body. It may be admixed to the aqueous monomer mixture either inthe form of a dry powder, or as a dispersion in water. During thepolymerization with the monomer mixture for spongy hydrogel the solublepolymer is interpolymerized therewith by grafting.

For thickening the aqueous monomer mixture to avoid its flowing downfrom the pad or from a plastic conveyor it is possible to use solublepolymers such as sodium polymethacrylate or similar, which are thenwashed out from the spongy hydrogel, if they were not bound byinterpolymerization.

When using the laminate according to the invention for dressing burntwounds or other comparatively large injured skin surfaces, it isadvantageous to provide the homogeneous layer with very fine holes whichmake possible the removal of the exudate without allowing access to thebacteria. Such holes may be made e.g. by treating the dry homogeneouslayer on the pad with a sharp brush, or by similar treatment of thefinished laminate with appropriate tools.

Instead of forming the thin homogeneous foil on a solid pad, it ispossible to use ready made polymer foil and to polymerize the spongyhydrogel thereto either directly or by means of an intermediate layer asmentioned above.

What we claim is:

1. Method of manufacturing hydrogel laminates consisting of a thin filmlayer of an originally soluble, noncrosslinked homogeneous hydrogelpolymer and a substantially thicker layer of a spongy cross-linkedhydrogel, comprising casting said homogeneous hydrogel polymer filmlayer on a solid pad, and thereafter a monomer mixture capable offorming said;spongy hydrogel layeris polymerized in situ oversaid filmlayer on said pad.

2. Method according to claim 1, wherein an intermediate layer is castonto said thin film layer by polymerizing a monomer mixture prior topolymerizing said spongy hydr'ogel thereon, said intermediate layerbeing less hydrophobic than said thin film layer but capable of'swellingit and forming a'bond between said layer and said spongy hydrogel.

3. Method according to claim 1, wherein the thin film layer is cast ontosaid pad by applying thereon a polymer solution which is thereafterdried by evaporating any solvent from said solution.

4. Method according to claim 1, wherein the monomer mixture for saidspongy hydrogel is first thickened by partial polymerizationdiscontinued prior to reaching the gel point, the polymerization beingcompleted subsequent to spreading the mixture onto the thin film layer.

- 5. A method of manufacturing hydrogel laminates according to claim 1,wherein said solid pad is first coated with a water soluble glycolmethacrylate polymer, operative to form a separating layer between thepolymers polymerized onto said pad.

6. Method according to claim 1, wherein the thin film layer containsvent holes.

7. Method according to claim 2, wherein said monomer mixture for saidspongy hydrogel contains a polymerization initiator and is spread oversaid intermediate layer onto which it is then polymerized.

8. Method according to claim 1, wherein said monomer mixture for saidspongy hydrogel is thickened by adding a suitable powdered fillerthereto.

9. Method according to claim 8, wherein the filler is a ground hydrogel.

10. Method according to claim 8, wherein the filler is a powderedsoluble, non-cross-linked glycol methacrylate or -acrylate polymer. H

11. A continuous process for carrying out the method according to claim1, wherein a thickened monomer mixture for said spongy hydrogel is castonto an endless conveyor containing a previously polymerized polymer ofsaid thin film layer, said conveyor being in a heated polymerizationzone containing an inert oxygen-free gas thereby completing thepolymerization.

12. A continuous process according to claim 11, wherein said endlessconveyor contains at least said two film, any subsequent filmpolymerized thereon being more hydrophilic than the previous one andbeing formed from a monomer mixture capable of swelling the nextsucceeding lower film. I

13. A continuous process according to claim 11, wherein said endlessconveyor is previously coated with a water soluble glycol methacrylatepolymer, operative to form a separating layer between the polymerspolymerized onto said pad.

References Cited UNITED STATES PATENTS 3,220,960 11/1965 Wichterle etal. 128-127 2,730,768 1/1956 Clarke 264-104 3,557,261 1/ 1971 Wichterle264-1 3,166,435 1/1965 Meier 264-212 3,362,942 1/1968 Monon 161-2473,686,024 8/1972 Nankee et al. 128-284 3,429,957 2/1969 Merten 264-2123,669,103 6/1972 Harper et al 128-284 3,069,728 7/1957 Mindick et al156-246 2,976,576 3/1961 Wichterle et al. 3-1 2,731,411 1/1956 Clarke264-104 CHARLES E. VAN HORN, Primary Examiner C. WESTON, AssistantExaminer US. Cl. X.R.

